1. Field of the Invention
The present invention generally relates to deposition of a metal layer onto a substrate. More particularly, the present invention relates to electroplating a metal layer onto a substrate.
2. Background of the Related Art
Sub-quarter micron multi-level metallization is one of the key technologies for the next generation of ultra large scale integration (ULSI). The multilevel interconnects that lie at the heart of this technology require planarization of interconnect features formed in high aspect ratio apertures, including contacts, vias, lines and other features. Reliable formation of these interconnect features is very important to the success of ULSI and to the continued effort to increase circuit density and quality on individual substrates and die.
As circuit densities increase, the widths of vias, contacts and other features, as well as the dielectric materials between them, decrease to less than 250 nanometers, whereas the thickness of the dielectric layers remains substantially constant, with the result that the aspect ratios for the features, i.e., their height divided by width, increases. Many traditional deposition processes have difficulty filling structures where the aspect ratio exceed 4:1, and particularly where it exceeds 10:1. Therefore, there is a great amount of ongoing effort being directed at the formation of void-free, nanometer-sized features having high aspect ratios wherein the ratio of feature height to feature width can be 4:1 or higher. Additionally, as the feature widths decrease, the device current remains constant or increases, which results in an increased current density in the feature.
Elemental aluminum (Al) and its alloys have been the traditional metals used to form lines and plugs in semiconductor processing because of aluminum""s perceived low electrical resistivity, its superior adhesion to silicon dioxide (SiO2), its ease of patterning, and the ability to obtain it in a highly pure form. However, aluminum has a higher electrical resistivity than other more conductive metals such as copper, and aluminum also can suffer from electromigration phenomena. Electromigration is believed to be the motion of ions of a metal conductor in response to the passage of high current through it, and it is a phenomenon that occurs in a metal circuit while the circuit is in operation, as opposed to a failure occurring during fabrication. Electromigration can lead to the formation of voids in the conductor. A void may accumulate and/or grow to a size where the immediate cross-section of the conductor is insufficient to support the quantity of current passing through the conductor, leading to an open circuit. The area of conductor available to conduct heat therealong likewise decreases where the void forms, increasing the risk of conductor failure. This problem is sometimes overcome by doping aluminum with copper and with tight texture or crystalline structure control of the material. However, electromigration in aluminum becomes increasingly problematic as the current density increases.
Copper and its alloys have lower resistivities than aluminum and significantly higher electromigration resistance as compared to aluminum. These characteristics are important for supporting the higher current densities experienced at high levels of integration and increase device speed. Copper also has good thermal conductivity and is available in a highly pure state. Therefore, copper is becoming a choice metal for filling sub-quarter micron, high aspect ratio interconnect features on semiconductor substrates.
Despite the desirability of using copper for semiconductor device fabrication, choices of fabrication methods for depositing copper into very high aspect ratio features, such as a 10:1 aspect ratio, 0.1 micron wide vias are limited. Precursors for CVD deposition of copper are ill-developed, and physical vapor deposition into such features produces unsatisfactory results because of voids formed in the features.
As a result of these process limitations, plating which had previously been limited to the fabrication of lines on circuit boards, is just now being used to fill vias and contacts on semiconductor devices. Metal electroplating in general is well known in the art and can be achieved by a variety of techniques. However, a number of obstacles impair consistent reliable electroplating of copper onto semiconductor substrates having nanometer-sized, high aspect ratio features. Generally, these obstacles deal with providing uniform power distribution and current density across the substrate plating surface to form a metal layer having uniform thickness.
Present designs of cells for electroplating a metal on semiconductor substrates are based on a fountain plater configuration. FIG. 1 is a cross sectional view of a simplified fountain plater. Generally, the fountain plater 10 includes an electrolyte container 12 having a top opening, a substrate holder 14 disposed above the electrolyte container 12, an anode 16 disposed at a bottom portion of the electrolyte container 12 and a cathode 20 contacting the substrate 18. The cathode 20 comprises a plurality of contact pins distributed about the peripheral portion of the substrate 18 to provide a bias about the perimeter of the substrate. The contact pins generally provide a higher current density near the contact points on the substrate surface, resulting in a non-uniform deposition on the substrate surface. The semiconductor substrate 18 is positioned a fixed distance above the cylindrical electrolyte container 12, and the electrolyte impinges perpendicularly on the substrate plating surface. Because of the dispersion effects of the electrical current at the exposed edges of the substrate 18 and the non-uniform flow of the electrolyte, the fountain plater 10 provides non-uniform current distribution, particularly at the region near the edges and at the center of the substrate 18 that results in non-uniform plating of the metal. The electrolyte flow uniformity at the center of the substrate 18 can be improved by rotating the substrate 18. However, the plating uniformity still deteriorates as the boundaries or edges of the substrate are approached.
Furthermore, the fountain plater 10 presents additional difficulties in substrate transfers because the substrate has to be flipped for face-down plating. Generally, substrates are transferred by robots having robot blades with a substrate supporting surface, and the substrates are transferred with the surface to be processed face-up. Preferably, the robot blade does not contact the surface to be processed to eliminate risk of damaging the substrate surface. Because the fountain plater 10 requires face-down processing, additional devices are required to flip the substrate from a face-up transferring position to a face-down processing position.
Therefore, there remains a need for a reliable, consistent copper electroplating technique to deposit and form copper layers on semiconductor substrates having nanometer-sized, high aspect ratio features. There is also a need for a face-up electroplating system that allows fast substrate processing and increases throughput. Furthermore, there is a need for an apparatus for delivering a uniform electrical power distribution to a substrate surface and a need for an electroplating system that provides uniform deposition on the substrate surface.
The invention generally provides an apparatus and a method for electro-chemically depositing a uniform metal layer onto a substrate. More specifically, the invention provides an electro-chemical deposition cell for face-up processing of semiconductor substrates comprising a substrate support member, a cathode connected to the substrate plating surface, an anode disposed above the substrate support member and an electroplating solution inlet supplying an electroplating solution fluidly connecting the anode and the substrate plating surface. Preferably, the anode comprises a consumable metal source disposed in a liquid permeable structure, and the anode and a cavity ring define a cavity for holding and distributing the electroplating solution to the substrate plating surface.
The invention also provides a substrate support member for face-up electro-plating. Preferably, the substrate support member comprises a vacuum chuck having vacuum ports disposed on the substrate supporting surface that serves to provide suction during processing and to provide a blow-off gas flow to prevent backside contamination during substrate transfers. The substrate support member also rotates and vibrates during processing to enhance the electro-deposition onto the substrate plating surface.
Another aspect of the invention provides a dual catch-cup system comprising an electroplating solution catch-cup and a rinse catch-cup. The dual catch-cup system provides separation of the electroplating solution and the rinse solutions during processing and provides re-circulating systems for the different solutions of the electroplating system.
The invention also provides an apparatus for delivering electrical power to a substrate surface comprising an annular ring electrically connected to a power supply, the annular ring having a contact portion to electrically contact a peripheral portion of the substrate surface. Preferably, the contact portion comprises annular surface, such as a metal impregnated elastomer ring, to provide continuous or substantially continuous electrical contact with the peripheral portion of the substrate. The invention provides a uniform distribution of power to a substrate deposition surface by providing a uniform current density across the substrate deposition surface through the continuous annular contact portion. The invention also prevents process solution contamination of the backside of the substrate by providing a seal between the contact portion of the annular ring and the substrate deposition surface.
Another aspect of the invention provides an apparatus for holding a substrate for electro-chemical deposition comprising a substrate holder having a substrate support surface and an annular ring electrically connected to a power supply, the annular ring having a contact portion to electrically contact a peripheral portion of the substrate surface. The substrate holder is preferably connected to one or more actuators that provide rotational movement and/or vibrational agitation to the substrate holder during processing to enhance deposition uniformity. Preferably, the substrate holder comprises a vacuum chuck having a substrate supporting surface, and an O-ring is disposed around a substrate supporting surface to seal the backside of the substrate from contamination by the processing solutions.